1. Field of the Invention
The present invention relates to a noise canceling headphone and a noise canceling earmuff, and in particular a noise canceling headphone and a noise canceling earmuff that allow a user to clearly hear desired external sound by compensating for external sound attenuated by a sound insulating characteristic of an ear cup while canceling external noise.
2. Related Background Art
A noise canceling headphone is known that realizes a noise-free situation by canceling unwanted sound (hereinafter referred to as “noise”) in ambient sound (hereinafter referred to as “external sound”) when wearing the headphone. The noise canceling headphone provides a speaker unit that outputs a musical signal input from an exterior and a noise canceling signal opposite to the noise in phase characteristic. A noise canceling earmuff is configured such that it only outputs a noise canceling signal without providing any musical-signal input mechanism, unlike the noise canceling headphone. The noise canceling headphone is similar to the noise canceling earmuff in generating and outputting the noise canceling signal for noise cancellation. This specification uses a noise canceling headphone for explanation of the present invention except where otherwise indicated. A noise canceling earmuff according to the present invention also provides features of the noise canceling headphone according to the present invention.
A conventional noise canceling headphone has the following configuration. When an electric power supply is applied, a circuit generates a noise canceling signal and a microphone included in an ear cup that configures a housing of the headphone collects external sound. The external sound collected by the microphone is converted into an electrical signal. A noise canceling signal generating circuit generates a noise canceling signal in response to the electrical signal. The generated noise canceling signal is output as a canceling sound from a speaker unit via an amplifier. The canceling sound exerts an effect of canceling noise reaching a user's ears. Accordingly the user can obtain a quiet circumstance where noise is canceled with the noise canceling headphone.
As explained above, when an electric power supply is applied, (during operation) the conventional noise canceling headphone generates and outputs a noise canceling signal that cancels the external sound collected by the microphone, thereby exerting a canceling effect on every kind of external sound. Even desired sound included in the external sound (for example, a human voice), therefore, is canceled. The user, however, may want to hear an ambient human voice in some cases, for example, when the user is on an airplane or a train while using the noise canceling headphone.
If the user tries to hear an ambient human voice while using the conventional noise canceling headphone, the operation of the noise canceling headphone needs to be stopped or the headphone removed. This manner of use is inconvenient for the user. As a solution to this problem, a noise canceling headphone is known that provides a so-called talk-through function in which “desired sound” such as an ambient human voice (hereinafter referred to as “target sound”) can be heard through a speaker unit provided in the headphone, while using the noise canceling headphone (for example, see Japanese Unexamined Patent Application Publication No. 2006-014307).
A noise canceling headphone that provides the talk-through function disclosed in Japanese Unexamined Patent Application Publication No. 2006-014307 provides a second talk-through microphone for collecting the target sound in addition to a microphone for collecting the external sound. The talk-trough second microphone can be operated (by switching operation) as required. If the user wants to hear an ambient human voice, the second microphone is operated, and at other times, the second microphone is not operated. Since the second microphone, however, collects the external sound during operation, if the human voice is louder than the external sound, the louder external sound is collected by the second microphone to be output through the speaker unit, this results in the risk of causing damage to the user's eardrum.
The noise canceling signal is not generated against the external sound collected by the second microphone. When the second microphone collects steady noise especially having a large number of bass frequency components, the steady noise is output through the speaker unit, which can cause interference with the noise canceling effect. As described above, the talk-through function of the conventional noise canceling headphone has a number of problems.